The present invention relates to a lithographic printing plate precursor. More particularly, the present invention relates to a lithographic printing plate precursor capable of performing plat-making with scanning exposure based on digital signals, having high-sensitivity and good press life, capable of providing printed matters free from stains, and capable of being mounted on a printing machine as it is for printing without development processing.
In recent years, various investigations have been made with respect to printing plates for a computer to plate system with the remarkable progress in the field. Among the investigations, for purposes of more rationalizing the plate-making process and solving the waste liquor treatment problem, lithographic printing plate precursors capable of being mounted on a printing machine as it is for printing without development processing after exposure have been studied and various methods have been proposed.
One method for eliminating processing steps is a method called on-machine development wherein an exposed printing plate precursor is mounted on a cylinder of printing machine and a fountain solution and printing ink are supplied on the printing plate precursor while rotating the cylinder, thereby removing a non-image area of the printing plate precursor. Specifically, according to the method, after exposing a printing plate precursor, the printing plate precursor is mounted on a printing machine as it is and processing is completed in an ordinary printing step.
The lithographic printing plate precursor suitable for such an on-machine development is required to have a photosensitive layer soluble in a fountain solution and an ink solvent and also a light-room handling property capable of development on a printing machine placed in a light room.
For instance, a lithographic printing plate precursor comprising a hydrophilic support provided thereon a photosensitive layer containing fine particles of a thermoplastic hydrophobic polymer dispersed in a hydrophilic binder polymer is described in Japanese Patent No. 2938397. In the patent, it is described that the lithographic printing plate precursor is exposed with an infrared laser to form images by coalescing the fine particles of the thermoplastic hydrophobic polymer by heat, mounted on a cylinder of a printing machine and subjected to the on-machine development with a fountain solution and/or printing ink.
Also, in JP-A-9-127683 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and WO 99/10186, it is described that after coalescing thermoplastic fine particles, a printing plate is prepared by the on-machine development.
However, the method of forming images by fusing of fine particles with heat as described above is accompanied with a problem that sensitivity is low or good press life is hardly obtained.
Therefore, an object of the present invention is to solve the problem, more specifically, to provide a lithographic printing plate precursor having a good on-machine developing property, high sensitivity and good press life.
Other objects of the present invention will become apparent from the following description.
As a result of the intensive investigations for attaining the above-described object, it has been found that the problem of the above-described prior art technique is solved by a lithographic printing plate precursor described below.
(1) A lithographic printing plate precursor comprising a hydrophilic support having provided thereon an image-forming layer containing a radical initiator, an infrared absorbing dye, and at least one component selected from fine particles containing a radical polymerizable compound having a structure represented by formula (I) shown below and microcapsules encapsulating a radical polymerizable compound having a structure represented by formula (I) shown below. 
wherein X1 and X2, which may be the same or different, each represents a halogen atom or a group connected through a hetero atom; Ra and Rb, which may be the same or different, each represents a hydrogen atom, a halogen atom, a cyano group or an organic residue; or X1 and X2, Ra and Rb, or X1 and Ra or Rb may combine with each other to form a cyclic structure.